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Summary
Dippers were monitored at around 70 winter roost sites in the River Teme Catchment from 1987 to 2000. This Project resumed this monitoring in 2006, and has also implemented a nest box scheme. By the start of the 2008 breeding season, boxes had been installed at 89 sites, and 24 of them were used.
Comparison of results obtained in 2006 - 2008 with those from the 1980s and 1990s show an overall decline in the number of Dippers present, a much greater decline on the lower reaches of the rivers than the upper reaches, and a deterioration in the condition of the birds (measured by average body weight).
This is attributed to a loss of food as a result of reduced quality of the rivers, primarily due to pollution from, and silting up by, agricultural activities.
However, more Dippers were found in 2008 than in any previous year, and this is attributed to improved breeding success in the upper reaches
of the rivers as a result of the nest box scheme.
Further long term monitoring of the Dipper population, and extending the nest box scheme, is recommended, to iron out any effect on the results from annual fluctuations, and the Environment Agency is recommended to analyse water sampling results from these river systems for the last 25 years, to ascertain if specific causes of the Dipper decline can be identified.
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Photo © John Swift
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Dippers
Cinclus cinclus
In the River Teme Catchment
2008
Dippers
Cinclus cinclus
In the River Teme Catchment
Conservation of Threatened Birds
AONB Sustainable Development Fund Project
2008 – 09
Supported by Natural England’s
River Teme Catchment Sensitive Farming Project
Dippers feed almost exclusively on larvae that live on the stony beds of rapids and fast flowing streams, and are never far from such waters. The rivers in the South Shropshire hills, particularly the East and West Onny, the Clun (and its tributaries, the Unk and the Folly Brook), and other parts of the Teme catchment, are the County strongholds. Dippers stay here throughout the year, and might be seen either bobbing up and down on the rocks in the middle of the stream, or flying low over the water.
Breeding usually starts early, in late February or early March, and many nesting pairs will attempt to raise two broods. Though some Dippers nest in natural cavities along the riverbank, others build nests on ledges under bridges, and they take readily to nest boxes located directly above the flowing water, where predators are unable to reach them.
They are very territorial, so nests are evenly spaced on each stretch of river.
Because Dippers are restricted to, and dependent on, food from the river, they are relatively easy to monitor. Pairs nesting along poor quality (acidic or silted up) streams tend to lay their eggs later, lay smaller clutches, raise smaller broods, and raise only one brood. The average size of the territory, breeding success, productivity and survival rate are therefore all good indicators of the water quality in the river.
As a result of the decline in the local population, Dipper is a Target Species in the Shropshire Biodiversity Action Plan (BAP).
The Habitat Action Plan for Rivers and Streams in the Shropshire BAP also makes reference to Dippers as a key indicator species, and includes a number of actions to reduce the enrichment of the watercourses by agricultural activities (which cause diffuse pollution which in turn affects the Dippers’ food supply). The use of sheep dip/cypermethryn still continues to cause problems with invertebrates on certain smaller watercourses, and this can have a direct and dramatic effect on Dippers’ food sources.
The Dipper population in the River Teme catchment was monitored extensively in the late 1980s and 1990s, up until 2000. Concern about the apparent recent decline led to a reinstatement of this monitoring, together with action to improve breeding success, initially through the Upper Onny Wildlife Group, beginning in 2005, and subsequently through the Upper Clun Community Wildlife Group, from 2007 onwards.
In 2006-08 the Project was part-funded by the Shropshire Hills AONB Sustainable Development Fund and the Upper Onny Wildlife Group. The Project was extended in 2007, and Natural England’s River Teme Catchment Sensitive Farming Project contributed additional funding in 2007-08 and 2008-09.
The Project consists of three complementary activities:-
This Report presents the Results for 2008, and also incorporates the 2006 and 2007 results for ease of comparison. Where appropriate it compares these recent results to those from the late 1980s and 1990s.
The Report and its content are public documents, and the results should be disseminated as widely as possible.
The work in 2008 repeated surveys carried out at many of the same sites, by the same surveyor (A.V. Cross), in 2006 and 2007, and in the late 1980s and 1990s.
During 2008 survey work was conducted at 87 past and present roosting sites for Dippers under road and footbridges over the Rivers Corve, Teme, Onny, Clun and Redlake and the Quinney Brook. Surveys were carried out on the nights of 10/11, 11/12, 17/18 and 25/26 October.
Bridges were visited in the hours of darkness and were inspected with a torch to see how many birds were roosting underneath them. Birds commonly roost on girders or in holes, drainpipes and other cavities below the bridges, including inside old Dipper nests and nestboxes.
The white breast shows well on a roosting bird and counts are a true measure of the numbers present.
Once a count had been obtained an attempt was made (under a BTO ringing licence) to catch as many of the Dippers as possible in order to ring or examine any ring already present. Catching is relatively simple as the birds sit tight and can be lifted off by hand or netted in a small hand-net. After ringing/examining the birds are replaced back under the bridge and the majority settle back down. A small percentage fly off and presumably then roost in bank side trees or under riverbanks.
A total of 87 sites were visited. The number has increased in recent years as the installation of nestboxes allows some bridges to be utilised as roost sites which were previously unusable. Birds were found at 52 of the 87 bridges visited with a total count of 129 birds – mean 1.48 Dippers / bridge checked or 2.48 Dippers/occupied roost. This compares with 2007, when 90 birds were found at 42 of the 70 bridges visited, and 2006, when 87 birds were found at 37 of the 67 bridges visited – mean 1.3 Dippers / bridge checked or 2.35 Dippers / occupied roost.
These figures when plotted against figures obtained in the 1980s and 1990s and fitted with linear trendlines clearly show a marked decline both in the number of Dippers per bridge and the number of Dippers present at occupied sites. The rate of decline in the number of Dippers per bridge checked is much steeper as many former bridge roost sites have now been abandoned completely.
These results are shown in Figure 1.
However, the total number of Dippers found in 2008 (129) was the highest ever. Although more roost sites are visited now, only 20 of the birds found were at new sites (i.e. 26 visited for the first time in 2006 or later, particularly to check if the new nest boxes were being used as roosts). The number found at old sites (109) was the second highest, and was higher than any year since 1988, when 117 were found. This is attributed to improved breeding success in 2008, partly due to calm river
conditions in the breeding season, but also, more importantly, to increased breeding success as a result of the nest box scheme. This is set out later in this Report.
Figure 1. Decline In Number Of Dippers At Bridge Roost Sites In Teme, Onny, Clun & Corve Catchments
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It was felt whilst undertaking the fieldwork that the bridges on the higher reaches of the rivers had maintained numbers quite well whilst the numbers found under bridges on the lower reaches seemed to have dropped much more. To investigate this further, the rivers were divided arbitrarily into lower and upper sections and the data for total number of birds found under all bridges on these sections over the past 20 years were plotted.
The divisions made are, on the Clun at Clun bridge, on the Onny below Horderley, and on the Teme below Knighton.
These results are shown in Figure 2.
It can be seen in the case of all three major Rivers (the Onny, Clun and Teme) that the number of birds on the lower reaches has indeed dropped considerably, whereas numbers on the upper reaches have increased. Numbers on the River Redlake have declined similar to those on the lower reaches of the other rivers despite, on face value, being more similar in river morphology to the upper reaches of these rivers.
Of the total of 129 birds observed roosting, 125 were caught for examination. The Age and Sex of the birds, together with the ratio of Males to Females, and of Adults to First-year birds, is shown in Table 1 below, together with the comparable figures for previous years.
The slight population bias towards males found in 2006 was partially repeated in 2008, but it is not reflected in the 2007 or “All Previous Years” figures.
The 2006 Report stated that the ratio of adults to first-years has remained the same throughout the study, suggesting that there has been little change in the breeding success and productivity of those birds which survive to breed, or the survival rate of newly-fledged young birds. However in 2007 this ratio was considerably less (1.08:1), compared to 1.23:1 in earlier years, indicating either an increase in mortality of adults, or an increase in newly fledged birds, due to increased brood size or an improved survival rate (or both). The ratio in 2008 reflected the same pattern.
Table 1. Age and Sex Ratios of Dippers Caught at Winter Roost Sites
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It will be seen that the average brood size of birds ringed in the nest was also lower in 2007. This was attributed to the abnormal extremely wet weather in May and June 2007, which affected breeding success. However, the average brood size found in 2008 was higher than in any previous year, so it might be expected that the ratio of adults to first-year birds would show a corresponding decrease. While there is little change in the ratio for 2008 compared to 2007, it is still considerably less than the 2006 or the “Up to 2000” ratio. In 2008 the high proportion of young birds is attributed to increased brood size, and an increasing number of breeding pairs, due to the nest box scheme (see Part 3 of this Report)
In 2008, 68 of the 122 different birds caught were already wearing rings, including 15 individuals (nine males and six females) ringed earlier in the year as nestlings. The average distance moved by these 15 newly-fledged birds was 4.34 kilometres. Three males and three females moved further than five kilometres. The nine males moved an average of 3.57 kilometres, while the six females moved an average of 8.02 kilometres.
In 2007, 37 of the birds caught were already wearing rings, including seven ringed earlier in the year as nestlings. The average distance moved by these 7 newly-fledged birds was 23.28 kilometres. The one male moved 15.98 kilometres, while the six females moved an average of 24.5 kilometres.
Forty-four of the birds caught in 2006 were already wearing rings including 13 ringed earlier in the year as nestlings. The average distance moved by these 13 newly-fledged birds was 7.55 kilometres. The seven males moved an average of 3.00 kilometres, while the six females moved an average of 12.85 kilometres.
It is known that the females of several species move further from the natal site than males. This is believed to be a natural selection mechanism to promote genetic diversity (i.e. reduce the risk of in-breeding).
The movements data has been submitted to the AONB Office for mapping.
Figure 2. Dippers Counted At Winter Roost Sites 1987 – 2008, By River Section
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All birds captured were weighed and the mean weight is shown in Table 2, together with the mean weight for similar age/sex groupings in 2007, 2006, and in all years before 2006 combined.
Table 2. Mean Weight of Dippers Caught at Winter Roost Sites
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The 2006 Report stated “All sexes/age classes have therefore shown a decrease in mean body mass of between 3.3 % and 2 %, suggestive of decreased food supply”. This decrease was even more pronounced in 2007, for all except Adult males. For them the mean weight went up slightly in 2007, compared to 2006, but it was still considerably lower than the “Up to 2000” figure. In 2008, the Adult males declined again, and first year females declined further, while first year males and adult females recovered slightly compared to last year, but did not return to the 2006 mean weights.
For all ages and sexes the 2008 mean weight is lower than the higher figure for either of the two previous years, and in all cases the 2006-08 figures are lower than the “Up to 2000” mean weights.
The deterioration in the condition of the Dippers,
as measured by mean body weight, therefore appears to be getting still worse.
The 125 birds caught for examination, described in Part 1 of this Report and summarised in Table 1, were all ringed.
Because of the importance of this Dipper Project, an increased effort was made to find nests and ring nestlings in 2008. The increased provision of nest boxes (see next section of this Report), together with monitoring of the boxes so the timing of ringing visits was more efficient, contributed to the large increase in nests visited. A total of one Adult Female, together with 33 broods containing 145 nestlings, were ringed at 30 different nest sites (a first and a second brood were ringed at three of these sites). The average Brood Size was 4.39. A nest with only one very large young remaining in it, and another site where two recently fledged young were caught and ringed, are not included in either these figures, or the analysis of the nest box scheme outlined in Part 3, as the total number of birds in these two broods may have been greater than the number ringed.
Data for 2008, and comparison with previous years, in shown in Table 3.
The mean brood size found in 2008 was much higher than that in 2006, and in the early period, and reversed the sharp decline found in 2007.
Table 3. Dippers Ringed at Nest Sites
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The incidence of different brood sizes, for 2008, 2007, 2006 and previous years is shown in Figure 3.
It can be seen that the incidence of large broods (five and six nestlings) declined in 2006 and 2007 compared with the earlier years, but recovered in 2008, perhaps due to the natural tendency of many species to increase productivity following years of poor breeding success. The impact of the nest box scheme, described later in Section 3 of this Report, has also helped.
Figure 3. Brood Size At Ringed Dipper Nests: 2008, 2007 and 2006 Compared With The 1980s And 1990s
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It should be noted that ringing effort has always been rather variable and fitted in around work on other species. Little data has been collected on the frequency and timing of second broods.
Dippers take readily to nest boxes. Each nest box must be located directly above the flowing water, in a position where predators are unable to reach it. Installation of boxes therefore increases the number of available nest sites (and potentially the number of possible territories, if suitable stretches of river would otherwise have no suitable nest site), and reduces the level of predation. Installation of boxes should therefore improve breeding success, and potentially increase the population. Regular inspection of the boxes also facilitates monitoring of the population and productivity, and helps determine the range of the species (i.e. which parts of the rivers are inhabited, and which are not).
Eighty-two bridges were surveyed by John Swift on the Rivers Onny and Clun, and their tributaries, in 2005-07. Sixty-five specially designed nest boxes for Dippers were made and installed at 55 different sites, mainly under these bridges. In 2007-08 a further 28 boxes were installed, 10 in the Upper Clun, and 18 on the Lower Clun as far as Clunbury. These latter boxes are intended to find out how far downstream the Dippers’ current range extends.
In 2008 – 09 a total of 24 boxes have been installed by the end of the calendar year 2008, and more are planned before the start of the 2009 breeding season.
In total 113 boxes have been installed at 89 locations in the Teme catchment since 2005 (not counting replacements), 13 of them at 10 sites during 2008-09. All of the boxes installed beforehand were monitored during the 2008 breeding season.
The relevant parts of this work have been carried out under the auspices of the Upper Onny Wildlife Group since 2005, and the Upper Clun Community Wildlife Group since 2007, and the work is also described in the respective Annual Reports of the two Community Wildlife Groups.
Initial survey work in the Upper Onny area in 2005 found seven Dipper nests. In three cases where the nest in each of two adjacent territories was found, the average nearest neighbour distance extended 1.4 kilometres along the river. Subsequently two boxes were installed in 2005, and the Upper Onny nest box scheme continued since then. Virtually every bridge in the area which is marked on the OS Map now has a Dipper nest box, and several bridges have two. A total of 40 boxes had been installed under 26 bridges prior to the 2008 breeding season (12 on the West Onny, 17 on the East Onny, two on Darnford Brook and nine on Criftin Brook). The possibility of installing boxes at other locations, for instance under fallen trees which span suitable rivers, or on private bridges in the area that are not shown on Ordnance Survey maps, will continue to be explored.
Monitoring of the 40 boxes in the Upper Onny, and known natural sites, continued in 2008
At least 11 of these pairs successfully raised young
This compares with 12 pairs found in 2007, and seven in 2006, with 10 nests in a box in 2007 and three in 2006.
In the Upper Clun area, 41 boxes had been installed under 30 bridges and other suitable structures prior to the start of the 2008 breeding season. In 2008
At least 10 of these pairs successfully raised young
This compares with nests of 13 pairs found in 2007. Ten of these nests were in boxes, and at least 7 of these pairs successfully raised young.
In 2007, seven nests were found along around 9 kilometres of river along Darnford Brook and the East Onny (including five in 2 kilometres, a neighbour distance of only 500m), and there were eight on this stretch in 2008. In both years there were three on 7.5 kilometres of the West Onny (including two nests 1.3 kilometres apart), but only one on Criftin Brook. The twelfth nest in 2007 was at Eaton, just past the confluence of the West and East Onnys, but in 2008 there were nests at two different sites at Horderly. It is likely that the scheme has increased the Dipper population in the area, although some if not all of the pairs in boxes will have moved from less satisfactory natural sites.
On the upper Clun, there were 20 nesting attempts in 2007 (i.e. including completed nests with eggs – the 13 referred to above - and seven other nests which were started but not completed), and 17 in 2008 (eggs were laid in at least 14 of the 17 nests). In 2007 there were eight nests on 7 kilometres of river in Newcastle and upstream from there (average neighbour distance = 1 kilometre) and seven in 2008, two more nests in Newcastle on the Folly Brook, and two more further up the Folly Brook in both 2007 and 2008 (total of four in around 4 kilometres on Folly Brook). No nests were found between Newcastle and Clun, but there were two in Clun itself (one outside the UCCWG area) in 2007, three on the Unk in 2007 and two in 2008, and one on the Mardu at Whitcott Keysett in both years, but another three on the Mardu in 2008.
Inspection of a map of these nest sites on the Upper Onny and Upper Clun shows regular spacing, but several gaps. There are boxes in some of these gaps, but it is not yet known whether Dippers are using other (natural) nest sites on these stretches of river, or they are really absent; and if so, whether this is due to absence of suitable feeding habitat. Now the number of nest boxes is close to the maximum possible, further work will be done to clarify this, and identify where Dippers are absent. The average neighbour distance on the different sections of river where Dippers are found will also indicate the relative quality of the habitat, which may reflect natural variations, but may also indicate sections of river which have been rendered unsuitable by farming activity, or other man-made influences.
The average brood size, and the total number of Dippers found at winter roost sites, were both the highest in 2008 since monitoring started in 1987.
While low rainfall in the spring of 2008 undoubtedly helped adults raise large broods, as finding food in the unswollen rivers would have been easier than usual, analysis of the number of birds in the 34 ringed broods suggests that the nest box scheme has also played a part in the population increase.
Of these 34 ringed broods, one in a nest box comprised only one very large chick. It is likely that other birds in the same brood had already fledged, so this brood is excluded from the analysis. Of the remaining 33 broods (average brood size = 4.39), 10 were in nest boxes. The average brood size of these 10 nests was 4.60, compared with 4.26 in the 23 other nests.
The nest boxes are mainly in the Upper Onny and Upper Clun areas, and all those used were in these two areas. In the Upper Onny, five broods raised in boxes averaged 4.60 compared with 4.20 for the other five broods. In the Upper Clun, the five broods raised in boxes included in the analysis averaged 4.60, compared with 4.33 for the other three broods.
Boxes provide more secure nest sites, and are preferentially selected by the Dippers. The boxes have only been available for three breeding seasons at most, and in only two parts of the whole area, but in that time five pairs have moved from previously known nest sites into boxes.
More importantly, seven pairs have moved into boxes on bridges that were previously unsuitable (there was no ledge or hole where a nest could be constructed). Some of these are almost certainly additions to the total breeding population, as the boxes allow new territories to be occupied in stretches of suitable river that have no available natural nest site. Although some of these pairs may have moved from natural sites, recollections from many years of monitoring Dippers suggest that the level of nest success in natural sites on riverbanks, or amongst boulders in the streams, are much more vulnerable to flooding or predation than those under bridges. It must be stressed that these more marginal sites are more difficult to find, and most of the nests included in the current study, apart from those in new boxes, were still in relatively secure sites under bridges. The average brood size actually found outside nestboxes in 2008 is therefore likely to be much higher than the overall average for the area. The boxes enable many more pairs to occupy secure nest sites, though there is no quantified data to calculate the actual increase in breeding pairs or improved breeding success as a result of this.
It therefore appears that nestboxes have contributed to the increase in population that has been found, through increased brood size and breeding success as a result of providing more secure nest sites, and through providing new nest sites which allow new territories to be occupied.
However, it must be stressed that nest boxes will not in themselves allow the re-colonisation of the whole of the Dippers’ former range. The birds are very territorial, and each territory requires a food supply as well as a nest site. Action is necessary by the statutory agencies to improve the river quality to restore the previously available feeding sites.
It is important that the Project continues to monitor breeding success in nest boxes in future years, to confirm that their apparent benefit does not just a reflect an unusual pattern in 2008 , and they make a real contribution to achieving the Shropshire BAP target to increase the population.
Deans, P., Sankey, J., Smith, L., Tucker, J., Whittles, C., & Wright, C. 1992. An
Atlas of the Breeding Birds Of Shropshire. The Shropshire Ornithological Society.
(Referred to as The Atlas (1992) throughout)_img_11.jpg)
Shropshire Biodiversity Action Plan, produced on behalf of the Shropshire Biodiversity Steering Group by Shropshire County Council November 2002, Revised and Updated November 2006
Management Plan 2004-2009 for the Shropshire Hills Area Of Outstanding Natural Beauty, published by the Shropshire Hills AONB Office July 2004
Cross, A.V. Monitoring the Dipper Population In The River Teme Catchment (Report of AONB SDF Project 2006 - 07)
Smith, L. Dippers In The River Teme Catchment (Report of AONB SDF Project 2007 - 08)
Smith, L. Lapwing, Curlew, & Other Wildlife In The Upper Onny Valley (Upper Onny Wildlife Group Survey Results & Reports 2005, 2006, & 2007, plus 2008 (In prep))
Smith, L. Upper Clun Community Wildlife Group Report 2007& 2008
Tony Cross carried out the survey work at 87 Winter Roost Sites in 2008, and ringed the Dippers there and at 22 nest sites, and provided most of the information (including the historical information) contained in this Report.
John Swift made and installed the Dipper nest boxes, and provided the remainder of the information contained in this Report.
The work is part of the Conservation of Threatened Birds in the Shropshire Hills AONB Project, co-ordinated by Leo Smith Ornithological Surveys and Consultancy.
This work was supported and funded by
This support is gratefully acknowledged.
Thanks are also due to:-
Leo Smith prepared this Report, which is printed on recycled paper
The Report and its content are public documents, and the results should be disseminated as widely as possible. Copies are being supplied to
Natural England
Environment Agency
Shropshire Hills AONB Partnership
Upper Onny Wildlife Group
Upper Clun Community Wildlife Group
Teme Catchment Fisheries Association
Shropshire Wildlife Trust
Shropshire County Council
Royal Society For The Protection Of Birds
Shropshire Ornithological Society
Birds In Counties
Project Workers
It is hoped that people on the Distribution List will pass the Report on to other relevant members of their organisations.
Additional copies are available from
Leo Smith
8 Welsh Street Gardens
Bishop’s Castle
Shropshire
SY9 5BH
01588 638577
leo.smith@dsl.pipex.com
There can be little doubt that, in the catchments of the Rivers Teme, Clun, Onny, Corve and the Quinney Brook, which drain a large part of the South Shropshire Hills and neighbouring Radnorshire and Herefordshire, there has been a steady decline in the number of Dippers roosting at traditional bridge winter roost sites over the last 20 years or so.
Despite fairly major changes in the bridge network in neighbouring areas, during the 20 odd years of this study little renovation work has been undertaken, and only two bridges have been altered to such an extent that they became unsuitable. Several other bridges were actually improved as potential roost sites by renovation / maintenance work, and the provision of nest boxes during 2005 - 08 has created further potential roost sites under previously unsuitable bridges.
Observations, although not well documented, also point to an abandonment of some of the traditional nest sites on the lower reaches of the rivers, especially on the Rivers Clun and Corve, despite the sites themselves appearing to remain suitable. Loss of habitat in the lower reaches of the rivers is confirmed by analysis of the numbers found at roost sites in the upper and lower reaches of the rivers, which shows that substantial declines have occurred in the lower reaches of all the rivers in this study.
These observations point to the causal factor of the decline being something other than the _img_12.jpg)
availability of suitable nest / roost sites._img_13.jpg)
Although the number of Dippers found at roost sites in 2008 was higher than in previous years, it is too soon to be sure whether this represents the beginning of a reversal of the decline, or is the abnormal result of an unusually successful breeding season. There is clear evidence that the nest-box scheme has improved breeding success in the upper reaches of the rivers, but provision of additional potential nest sites will be of no help to birds in stretches of river where there is no food.
The impression derived when visiting the roost sites, especially on the lower reaches of the rivers, is of a river-bed which is now subject to a much greater growth of slimy algae, presumably due to nutrient enrichment from agricultural run-off, than it was during the late 1980s early 1990s. Some silting up also appears to have occurred. Hopefully routine Environment Agency water sampling has recorded the increase in nutrient loading, and silt, in these rivers.
Much concern has also been expressed about the possible effects of sheep-dip chemicals such as cypermethryn on aquatic invertebrates, which would further reduce Dippers’ food supply.
The reduction in mean body mass of all age and sex categories of Dipper caught during 2006-08, in comparison to those caught in earlier years, strongly suggests a decline in food supply_img_14.jpg)
. This has almost certainly contributed to the decline, and is of great concern. In addition, the number of fish observed in the torchbeam whilst searching under roost bridges also appears to have declined greatly during the same period. Although the number of Dippers found in 2008 was higher than in previous years, their survival through to the next breeding season is threatened by the relatively poor condition in which they start the winter.
It therefore appears that the nest-box scheme, coupled with favourable weather conditions in _img_15.jpg)
the breeding season in 2008, has led to an increase in the population, but the poor condition of _img_16.jpg)
the rivers, particularly in the lower reaches, has led to a contraction of range, and reduced the _img_17.jpg)
condition of the surviving birds. _img_18.jpg)
Further monitoring of the Dipper population in these catchments is therefore necessary_img_19.jpg)
, through a combination of continued roost counts and the ringing of birds present, together with extending the nest box scheme, and visiting nest sites and ringing the birds there too. This will facilitate a much more systematic study of the range, neighbour distances, breeding success and productivity. In particular, extending the nest box scheme into the lower reaches of the rivers will confirm whether or not these waters have become unsuitable. All this information will help discover the causal factors in the recorded decline. As a key indicator of the health of the aquatic ecosystem in these upland rivers, addressing the factors responsible for the decline of the Dipper will help to restore these increasingly barren waters to their previously healthy state, in accordance with the targets in the Shropshire Biodiversity Action Plan.
This work should be repeated for several more years_img_20.jpg)
to remove any random annual fluctuations in the counts, particularly insofar as it might affect the relative population trends in the upper and lower reaches of the rivers, the anomalous trend on the River Redlake, and the high counts in 2008. A rigorous statistical analysis of the data should also be carried out, to clarify the apparent trends identified above.
In addition, the Environment Agency is recommended to analyse water sampling results from _img_21.jpg)
these river systems for the last 25 years_img_22.jpg)
or so, to measure nutrient enrichment and pesticide concentrations from agricultural run-off, and silting up, and assess whether these or other factors are responsible for the overall decline in the Dipper population, the variation in the decline between the upper and lower reaches of the rivers, the apparently anomalous trend on the River Redlake, and the observed reduction in mean body weight.
Tony Cross
John Swift
Leo Smith
February 2009